Introduction and seeking advice on 48V battery bank replacement

OffGridMarty · April 2020

Hello all, we've been reading this forum for awhile and have found the information invaluable, thank you for keeping it going. I'll start by listing our system components:

Morningstar Tristar TS-MPPT-60 with a TS-M-2 digital meter. The temperature sensor and voltage sense are in use. We have updated the firmware and settings ourselves with the MSView software.
LG 400W panels x4 in a 2S2P configuration.
Latronics LS-3548 3500W inverter.
Enersys SBS190F SLA 12V batteries x4 connected in series for 190AH @48V.
Victron BMV-700 monitor.

We (2 adults) have been off grid for about 5 years and have been content with the system. We are located in country New South Wales, Australia. Our power usage is very low and we rarely dip below 80% SOC since getting the 1600W of panels last year. Before that we only had 500W of panels and the batteries would often not see float for days at a time in the winter months. We do not us a generator with the system.

The batteries have been at 12.2ish volts in the mornings lately. Still ok by me but will need replacing in the next year or so I think. I only paid $1200 for them new, about 5 years ago, so feel that they have been a good value. I would consider getting another set for a similar price but they are no longer available locally.

Unfortunately my journey with learning about off grid and electricity has been like learning another language that I still don't fully grasp. I'm seeking help with figuring out what size battery bank would be best for this system. 190AH has been ok, a bit more would be better, but I don't want to oversize the battery bank for the system. Any help is appreciated and I will endeavor to answer any questions promptly.

Thanks much,

Marty

mcgivor · April 2020

Welcome to the forum Marty

After reading your post it would appear that whoever did the initial load calculations did a reasonable job considering the life you have achieved from the battery. Looking into the future for replacement options is better than having to make last minute decisions, you are correct that having an oversized bank would put you back in the position you were with the smaller array.

One of the things that drives the array size is the fact that lead acid batteries charge relitivly slow due to the absorption cycle, once the constant current bulk stage transitions to constant voltage absorption stage the current begins to drop off, making the charging system as a whole less efficient. During periods of cloudy weather it's easy to get behind on the required charging meaning longer charging times are required in the days following such an event, this is an important thing to understand and where generators are helpful.

As you are aware LA batteries are best kept at a higher than 50% SOC to extend cycle life, they do die of old age, some say 5-7 years, so having a bank larger than needed may result in less efficient use due to lost potentia,l if they expire without being used.

My experience with FLA was not particularly good, the primary reason was heat, being tropical the battery temperature rarely drops below 30°C often going above 40°C, my solution was to switch to LiFePo4 which are far happier with temperatures above 25°C. Three years is all I could get before things began to decline. With the LiFePo4 the battery temperature rarely exceeds 30°, only on hot day, like today 45°C do they climb to a maximum recorded 34°C.

There are other advantages which came along with the change in chemistry, much faster charging, no I'll effects in partial states of charge, higher efficiency which equals less heat gain, greater use of the nominal capacity, extremely low maintenance, smaller footprint, and lower mass. The disadvantage was higher initial cost, but this is offset by the longer life expectancy.

When I did the switch I replaced with the same nominal capacity but the very conservative increase usable capacity is 300 Ah compared to 200Ah. Needless to say I'm extremely happy I made the switch for temperature reasons and would never consider LA again, as long as freezing temperatures are not an issue, I would highly recommend the chemistries consideration.

Ivor

OffGridMarty · April 2020

mcgivor said:
After reading your post it would appear that whoever did the initial load calculations did a reasonable job [...]

Hi Ivor, thanks for the reply. There weren't really any calculations done. There were 500w of panels on the roof when we bought the place and a few old dead car batteries badly connected to them. We pulled out the batteries and wiring then installed the rest of the system ourselves (with advise from an electrician). The batteries were found by calling a number in a local newspaper advertisement that was selling old telecom batteries out of a garage. The fellow selling them also had a pile of new Enersys 190F batteries. I'm not so sure he should have had them or been selling them but that's history.

We do get high temperatures here, sometimes over 40C for a few days in a row, the highest I've seen is 45C, the lowest is 0C but that doesn't happen often. I've have read about LiFePo4 and would like to try them but the BMS required and such makes them intimidating to me. I think I just need to do more research into them. Thank you for the info and your opinion

softdown said:

[...]Your latitude is ~32 and possibly inland according to the map. Heat being a greater challenge than the cold I would guess.[...]

Hi Softdown, thanks for the the info and your opinion. We are very far inland and heat is more of an issue than cold. I've had a look and we are closer to 35.5 degrees latitude. My wife and I have learned a lot about energy conservation when only having the 500w of panels. We live very frugally and "live with the sun" as they say.

I do have the means to move around a forklift battery, I'll look into those as well, thanks for the suggestion. I'm still curious as to the optimal AH rating of battery bank that would suit the 1600w of panels? Or does it not work like that?

OffGridMarty · April 2020

softdown said:

We have a couple members well known for chugging out the theoretically ideal math figures. I suspect one will will show up within 48 hours. I'm a simple man, this is how I generally figure things: Gets lots of sturdy panels and big batteries with big copper cables. Cross fingers and conserve energy when it is realistic. Keep a system voltage read out handy and look at it several times/day.

What angle are your panels mounted at? Are the new panels similar in characteristics to the old panels? They should be, it is very important really.

I'm a simple guy too, and as I said before, after years of reading about off grid and electricity, I still barely understand the math and such. The TS-M-2 digital meter, BMV-700 & the Tristar Live View web page portal thing make it real easy to keep an eye on the system voltages.

I have no idea on what angle the panels are at, I really should know that, I'll try to get up there and find out soon. What do you mean by characteristics? We replaced the old 5x100w panels with the new 4x400w panels (our north roof space is very limited). The new panels specs can be seen here. I have no idea of the specs on the old panels as there are no markings or labels to be found. They appear to be cheap chinese 12v panels and I had measured their Voc to be about 8 amps.

Edit: I think i know what you mean, the panels characteristics (or specs) should be very similar to the old panels if adding them to the array. We did not add the new panels to the old ones, but replaced the entire array with the new ones.

mcgivor · April 2020

@OffGridMarty Building a DIY bank is something not everyone is comfortable doing, personally I did so for the educational value, undestanding how something works is intriguing to me. There are however pre built batteries with on board BMS, the next time I purchase I'll go that route as it actually works out cheaper, here is a $2200 48V 200Ah one, as an example, https://www.aliexpress.com/item/32936515660.html?spm=a2g0o.detail.1000013.5.36093d81ys3dtA&gps-id=pcDetailBottomMoreThisSeller&scm=1007.14977.161853.0&scm_id=1007.14977.161853.0&scm-url=1007.14977.161853.0&pvid=4af78904-2db8-48a0-80dd-a1d0b8bf41c0&_t=gps-id:pcDetailBottomMoreThisSeller,scm-url:1007.14977.161853.0,pvid:4af78904-2db8-48a0-80dd-a1d0b8bf41c0,tpp_buckets:668#0#131923#17_668#808#5965#274_668#888#3325#3_668#2717#7567#972 not something I would recommend or suggest, just a random one off search, there are many to choose from. Many will custom build to personal requirements such as maximum charge/discharge rates for the BMS. There are also higher end name brand offerings but the prices are substantially higher.

Ivor

OffGridMarty · April 2020

mcgivor said:

@OffGridMarty Building a DIY bank is something not everyone is comfortable doing, personally I did so for the educational value, undestanding how something works is intriguing to me. There are however pre built batteries with on board BMS, the next time I purchase I'll go that route as it actually works out cheaper, here is a $2200 48V 200Ah one, as an example, not something I would recommend or suggest, just a random one off search, there are many to choose from. Many will custom build to personal requirements such as maximum charge/discharge rates for the BMS. There are also higher end name brand offerings but the prices are substantially higher.

Hot dog Ivor, that looks tasty. I'd gladly get something like that at 200ah and that price if I knew it was quality and would work well with the rest of my system. I don't think I would attempt to build my own, I don't have enough time and/or knowledge.

mcgivor · April 2020

There are many suppliers, the seller I purchased from had excellent service and support though I'm not sure they have pre built banks, but I could ask. Do some searching and read reviews as a rough guide, I've dealt with many Alibaba Aliexpress vendors with very little dissapointment, only one I had trouble with but it wasn't a serious problem.

mcgivor · April 2020

That should have read I'm not recommending as I'm not a salesperson trying to promote one brand or another, for all I know they are reputable, If I have had dealings with a vendor with positive results and is not in conflict with our host, by geographic or product availability , I may provide information, but am reluctant to do so.

Today I did contact the vendor who supplied my cells, they don't supply pre assembled batteries only single cells, his offering was 200Ah CALB cells at $135 each so $2160 for a 48V 200Ah set, they can also supply the required BMS but that's not what the OP is interested in despite being very easy to build, IMHO.

BB. · April 2020

Sounds like you are doing fine... If you don't drop below ~75% state of charge on a daily basis, and only hit 50% SoC not too many times a year (and get recharged >80% SoC pretty quickly)--I don't really need to do much math here.

As you have seen, solar panels are what make the energy. Batteries are there only for short term storage (power over night, perhaps a couple days of bad weather in a row--Then a genset to help and/or cut down on power usage until the sun is shining again).

A larger battery bank does not "generate" more power... At this point, if you are happy with the battery bank performance (it gives you a couple days of storage, can start your loads, like refrigerator, well/water pump/etc.), then it sounds like you are at the "optimum" price/performance point.

And if you do get the urge to add "something"--Adding panels would be my suggestion. Your batteries will thank you.

Other than going to Li Ion batteries--I would be suggesting 2x 6 volt @ 200 AH "golf cart" batteries instead (or other 6/4/2 volt batteries instead of 4x 12 vol t batteries in parallel).

You have 4x 12 volt @ 190 AH Sealed Lead Acid batteries in parallel--That is 12 volts @ 760 AH battery bank... Not a small battery bank.

SLA batteries, you cannot use a hydrometer to measure state of charge. Voltage measurements are, at best, just rough estimates.

But since you are getting good life out of them, FLA batteries (which are cheaper, bit more rugged, but require more maintenance), that is just a choice you would have to make (same for Li Ion battery choices). 5-7 year life for daily off grid use of SLA batteries--You are getting good life out of them (especially if a warm climate/battery room--Heat is hard on batteries in general, and lead acid specifically).

At that size of battery bank (760 AH), you are about at the point where you may (someday) want to think about changing to a 24 (or 48) volt battery bank... 12 volts @ 760 AH stores the same amount of energy as 24 volts @ 380 AH (just change to 2x series by 2x parallel for the batteries you have). This reduces the current in your wiring by 1/2 (less voltage drop), and if you ever want to add more storage/solar panels, the higher voltage can save on wiring and charge controller costs (a 60 amp controller at 24 volts will manage 2x more solar panels vs 60 amps at 12 volts).

Just to mark a point in the sand, generally suggest 10%-13% rate of charge for full time off grid systems (all things being equal--which is rarely true):

760 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,431 Watt array nominal
760 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,861 Watt array typical "cost effective" maximum

You have a very nicely balanced system at this point (1,600 Watt array). About the only big concern I have is the 3,500 Watt AC inverter. Realistically, from a 12 volt battery bank it is difficult to pull much more than ~1,800 Watts... Fully loading a 3,500 Watt inverter is a heck of a lot of current:

3,500 Watts * 1/0.85 AC inverter eff * 1/10.5 volt battery cutoff = 392 Amps @ ~12 volt battery bank

If you needed more power (well pump, AC power tools, etc.) than ~1,800 Watts, I would suggest a 24 volt battery bank.

And the present battery bank, I would suggest that maximum inverter rating would be closer to:

760 AH * 12 volts * 0.85 AC inverter eff * 1/5 hour discharge rate (take battery bank to 50% in ~2.5 hours) = 1,550 Watt inverter rating

If this system is meeting your needs today--I cannot disagree... But you are in the position of what to do next--And these are some options (24 volt battery bank, larger battery bank capacity, larger/smaller AC inverter, larger array, etc.)....

-Bill

OffGridMarty · April 2020

BB. said:
You have 4x 12 volt @ 190 AH Sealed Lead Acid batteries in parallel--That is 12 volts @ 760 AH battery bank... Not a small battery bank [...]

Thanks for the reply BB, and apologies for not being clearer on this aspect, but my battery bank is connected in series for 48V. My Latronics LS-3548 inverter has an input range of 42-68V. I've just gotten into my job so can't reply to everything right now, will try to later.

BB. · April 2020

OK... That is better. However, the battery bank is still on the "small side" for a 3.5 kWatt AC inverter.

But it does depend on how much power you really want to draw (AGM batteries can discharge a C/1 or even higher rates).

But for an average off grid system, being able to discharge the battery bank in less than 1 hour is not usually very useful (takes several days to fully recharge).

I missed your title with did say you had a 48 volt battery bank. The rest of the calculations are typically done in Watts and WattHours, so they still are accurate.

-Bill

OffGridMarty · April 2020

BB. said:

OK... That is better. However, the battery bank is still on the "small side" for a 3.5 kWatt AC inverter.

But it does depend on how much power you really want to draw (AGM batteries can discharge a C/1 or even higher rates).[...]

I agree Bill, the inverter is oversized but we bought it in hopes of having a bigger system one day. We never actually utilize the 3500w it can provide and don't use multiple high watt appliances at the same time.

OffGridMarty · April 2020

softdown said:

Buying a complete unknown such as McGivor's example? [...]

Softdown, apologies if I came across as foolhardy enough to be buying the first thing someone posts. I wouldn't do such a thing without thorough research. I will take the time to research before buying anything, which is why I included the "if I knew it was quality and would work well with the rest of my system" bit.

We do have an aussie version of Craigslist called Gumtree and I do keep my eye on it for deals but they are few and far between. The past few days I've been looking at Gumtree, Alibaba and other sites looking at Trojans, golf cart batteries, LifePo4s, L16s and many others. Last night my wife and I spent a few hours on youtube looking at people constructing their own LifePo4 packs with balancers. I believe it is something we are capable of, given enough research and advice.

mcgivor said:

[...]but that's not what the OP is interested in despite being very easy to build, IMHO.

After watching some youtube last night I feel that I might be interested now. My wife and I are very handy when it comes to assembly, wiring, soldering and such. We hooked up all our solar system components ourselves (except for the inverter, legally that had to be done by an electrician) and it's been running fine for 5 years. I'm just a bit intimidated by getting the right components, the math involved and not dumping big money on making mistakes. I'll keep researching, and thanks for contacting you supplier!

mcgivor · April 2020

@OffGridMarty This site http://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/ which is devoted to marine applications though the fundementals remain the same, I found to be very helpful when I began, it contains most of the information needed, well worth reading.

OffGridMarty · April 2020

Ivor, thanks much for the link

OffGridMarty · April 2020

OffGridMarty said:

softdown said:

Buying a complete unknown such as McGivor's example? [...]

Softdown, apologies if I came across as foolhardy enough to be buying the first thing someone posts. I wouldn't do such a thing without thorough research. I will take the time to research before buying anything, which is why I included the "if I knew it was quality and would work well with the rest of my system" bit.

Did this come across as condescending? I didn't mean it to. I have a hard time conveying myself online without coming across as an a-hole. I just meant to say that I try to research everything before buying and that I'm not to type to jump blindly into large purchases.

Dave Angelini · April 2020

If you are researching keep in mind that if insurance to your home is important, many batteries, when sold as individual cells may have UN 1973 or the US UL 1973 listings. When combined into a bank with a BMS they do not. This keeps them safer from lawsuits here.

If there were a loss at the home, even if not directly attributed to the power system, you could have an uncovered loss. We no longer have insurance here do to wildfires. Even though we have prepared for them, California is alot like Australia in that there is too much risk. It is yet another reason to stay with one make of equipment systemwide.
Good Luck!

Introduction and seeking advice on 48V battery bank replacement

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